Eyesafe Q-switched laser

ABSTRACT

An eyesafe, Q-switched, laser system for target identification, ranging, and gated viewing purposes features a Resonant Pumped Erbium (RPE) laser, which exhibits a longer storage lifetime that minimizes the number of diodes needed to pump optical parametric oscillators (OPOs) or Raman converters. The RPE laser is in band to I 2  devices.

RELATED APPLICATION

[0001] Applicants hereby claim benefit of the priority date for U.S.Provisional patent application Serial No. 60/201,645, filed May 2, 2000.

FIELD OF THE INVENTION

[0002] The present invention relates to lasers, and more particularly,to high energy storage, high gain, eyesafe, Er lasers useful formilitary and commercial applications.

BACKGROUND OF THE INVENTION

[0003] A need has emerged for the development of high gain, high energyQ-switched lasers for military and commercial applications that operatein the eyesafe region (λ˜>1.5 microns). The military requirement is theaugmentation of fire control systems with a system that can identify thetarget (Target ID). The Target ID function is achieved with anelectro-optic sensor that illuminates the target with high short pulseeyesafe radiation (1.5 microns) and generates a real time image with thereturn radiation using a gated image intensifier or an array of highspeed photo to diode.

[0004] Commercial applications of high gain eyesafe lasers include thedevelopment of free space communication nodes of conventional fiberoptic networks. Free space networks which are capable of transmittingand receiving the data of fiber optic communication links candramatically reduce the capital cost of point to point fiber optic cableinstallation. Free space communication would be enabled by thedevelopment of high gain Er amplifier modules that would amplify theoutput of existing fiber networks to achieve reliable free spacecommunications. Free space communications must operate in all weatherenvironments requiring the transmission of high power; hence the highgain requirement.

[0005] Conventional technology utilizes Yb sensitized Er glass or Erfiber lasers. This technology has found wide military (eyesaferangefinders) and commercial (fiber communications) application.However, these technologies (Yb-Er glass and fiber lasers) cannotprovide the high gain, high energy storage properties needed forgeneration of moderate to high energy laser radiation for militaryapplication or the high needed for free space communicationapplications. The problem with the Yb-Er glass is the requirement ofhigh population densities in the upper level, which densities are neededto achieve gain and high energy. High sensitizer (Yb) concentrations areneeded to absorb the pump energy, and transfer it to the upper laserlevel. The high population densities at the upper laser level generallyback transfer, also known as upconversion, to the sensitizer, thuslimiting the amount of energy that can be stored. Gain is proportionalto stored energy, stored energy to laser output. Yb-Er glass technologycannot provide high energy laser output or high gain.

[0006] An alternative technology for military applications is the use ofdiode pumped Nd lasers driving non-linear devices which shift the onemicron laser output to the eyesafe region (λ>1.5 microns). Thistechnology is capable of high Q-switched eyesafe energies but is verycostly to implement. High cost is the result of the short storagelifetime of Nd lasers (200 μs). Typical diode bars emit 100 watts ofpeak power. The energy stored in the Nd laser is proportional to theproduct of the diode peak power×the storage lifetime of the lasermaterial.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention there is provided aneyesafe, Q-switched laser for target identification purposes or an Ergain medium for amplification of Er fiber communication links. Theinvention comprises a resonant pumped Er laser configured as aQ-switched oscillator or multipass amplifier.

[0008] The eyesafe Q-switched laser or multipass amplifier consists ofunsensitized Er, having a lifetime of ˜10 milliseconds for a 1.5 microntransition. The range of concentrations is approximately between 0.1 and2.0% of the active ion. The oscillator material can comprise erbiumdispersed in a number of crystalline or glass media. The laser systemhas the following performance values:

[0009] Energy/pulse: between approximately 250 and 300 mJ

[0010] Wavelength: ˜1.5 microns

[0011] Pump source: ˜30 to 60 W at 1.5 microns for ˜10 ms

[0012] Gain: gain coefficient ˜1 cm⁻¹

[0013] It is an object of the present invention to provide a low cost,Q-switched laser for target identification, ranging, and gated viewing.

[0014] It is another object of this invention to provide a means ofamplifying the output of a fiber communications link.

[0015] It is another object of this invention to provide a ResonantPumped Erbium (RPE) laser, which exhibits a long storage lifetime,thereby minimizing the number of diodes needed to achieve high storedenergy and high gain.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] A complete understanding of the present invention may be obtainedby reference to the accompanying drawings, when considered inconjunction with the subsequent detailed description, in which:

[0017]FIG. 1 illustrates a schematic diagram of an upconversionmechanism avoiding cross relaxation;

[0018]FIG. 2 depicts a schematic diagram of an upconversion mechanismfor a Tm-Ho YAG laser system;

[0019]FIGS. 3a and 3 b are graphical views of energy storage lifetimeversus Pulse Repetition Frequency for Tm-Ho and Ho YAG laser systems,respectively;

[0020]FIG. 4 illustrates a schematic view of upconversion mechanisms inYttrium-Erbium glass; and

[0021]FIG. 5 depicts a schematic view of a fiber laser, pumped, eyesafesystem in accordance with this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] The present invention features an eyesafe, Q-switched laser fortarget identification, ranging, and gated viewing purposes. The lasersystem comprises a Resonant Pumped Erbium (RPE) laser, which exhibits alonger storage lifetime that minimizes the number of diodes needed topump the optical parametric oscillators (OPOs) or Raman converters. TheRPE laser is in band to I² devices.

[0023] Current technology utilizes diode pumped Nd bars drivingnon-linear devices to achieve eyesafe output for gated imaging. Thephysical calculations for determining the number of diodes that isneeded for the system is easily obtained. Each diode produces “P” wattsfor some time duration, “T”, where T is determined by the fluorescencelifetime of the laser material. For example, for ND:YAG, T=200 μs.Therefore for 50 W bars, the total energy of each diode bar is dictatedby the equation:

E _(o) =P _(T)=10 mJ  (Eq. 1)

[0024] Diode to Nd conversion efficiency is 25%, so each diode barproduces 2.5 mJ of Q-switched, 1 micron radiation. At 30%, conversion of1 micron to eyesafe, each diode bar produces approximately 1 mJ ofeyesafe output.

[0025] Spontaneous emission rates of atomic transitions exhibit a V 3dependence on energy above the ground state, times the oscillatorstrength of the transition. Therefore, a transition at λ˜2 microns abovethe ground state exhibits (2/0.8)³=15 times the storage lifetime of theNd transition at 0.8 microns above the ground state, provided bothtransitions are of the same oscillator strength.

[0026] Realization of long storage lifetime in infrared transitionsrequires that two other conditions be met:

[0027] a) the state must be free from lower lying states thatnon-radiatively quench the excited state (multiphoton relaxation); and

[0028] b) the state must not be subject to decay modes that depopulatethe state at high inversion levels (“upconversion”, or energy transfer).

[0029] Three ions meet the (a) condition, all exhibiting purelyradiative transitions in fluoride hosts:

[0030]⁵l₇ (Ho) at 2 microns,

[0031]³F₄ (Tm) at 1.9 microns, and

[0032]⁴I_(13/2) (Er) at 1.5 microns.

[0033] The aforementioned condition (b) is more subtle. In order toavoid upconversion at high stored energy densities, it is necessary toavoid “cross relaxation”. This phenomenon is illustrated in FIG. 1.

[0034] Two ions, initially excited to states BIT and “A”, cross relax,leaving one ion in “D” and the other ion in “C”. The de-excitationoccurs at a rate proportional to:

ηA·ηB·f(V _(A) −V _(B))  (Eq. 2)

[0035] where ηA and ηb are the population densities of states A and B,respectively, and f is a function of the energy mismatch between thetransitions.

[0036] Referring to FIG. 2, levels and cross relaxation mechanisms areshown for the Tm-sensitized Ho laser. The ⁵I₇ upper laser level is fedby the cross relaxation mechanism shown; as the ′I population builds theupconversion mechanism dominates. The consequence of this is a rapidlydecreasing storage lifetime with pump intensity, or stored energy(gain).

[0037] Referring to FIG. 3, the measured Tm-Ho:YAG pulse energy isillustrated as a function of Pulse Repetition Frequency (PRF). The laseris pumped to a high gain and the effective lifetime is inferred from theenergy/pulse vs. PRF. From this data, a lifetime of 190 μs is achieved,in marked contrast to the spectroscopic lifetime of 5 ms for the ′Itransition.

[0038] It has been demonstrated that in unsensitized Ho:YAG in diluteconcentration, upconversion is negligible even at high gain. FIG. 3alone shows the pulse energy vs. PRF of Ho:YAG, from which a lifetime of3 ms is inferred. Ho:YAG is being developed for high pulse energies in3D Lidar and remote sensing applications. A 10 mJ/diode bar ofQ-switched, two micron output has already been demonstrated. However,this two micron transition is out of band to I² devices.

[0039] Referring to FIG. 4, the level diagrams in the case of Tm-Ho, asthe population of the ⁴I_(13/2) state, illustrate increased upconversionvia the occurrence of near resonant cross relaxation. The spectroscopiclifetime of approximately 10 ms in fluorides is rapidly reduced by therate for this cross relaxation process to the extent that it becomes ofthe order of the pumping rate. At this point, the population density is“clamped”; stronger pumping does not increase stored energy. Yb-Er glasslasers are notoriously low gain (i.e., low energy storage) devices.

[0040] The invention considers dilute, unsensitized Er doped crystals(YLF, YAG, Yttrium Orthoaluminate [YAIO]) to be a means of generatinghigh pulse energy for diode pumped, eyesafe lasers.

[0041] In unsensitized, low-concentration, Er crystals, upconversion isinherently a weak process; the spectroscopic lifetime is realized atuseful gain levels. This means that each diode bar will store:

[0042] 10 ms/0.2 ms=50 times the energy of an Nd laser.

[0043] Use of unsensitized Er lasers at 1.5 microns has not beenattempted in the past because of difficulties in pumping the firstexcited state of Er around 1.5 microns.

[0044] With reference to FIG. 5, Er fiber lasers, 1.5 micron diode bars,and diode pumped Yb-Er glass, in long pulse lasers, are illustrated inthe system of this invention. FIG. 5 is a schematic view of a 250 mJ,Q-switched, eyesafe laser system, arrow 10. The Pump Source for theErbium-Yttrium Lithium Fluoride laser 14 comprises two 30 W Er fiberlasers 12 emitting at 1.5 microns. Sixty watts of pumped energy storedfor 10 ms correspond to 600 mJ of pumped energy.

[0045] At 50% net coupling, an extraction efficiency is obtained inexcess of 250 mJ. This is in stark contrast to a system featuring a 400diode bar pumped Nd:YAG OSC AMP-AMP (1J) pumping a potassium titaniumoxide phosphate (KTP) or a potassium titanyl arsenate (KTA) OpticalParametric Oscillator-Optical Parametric Amplifier (OPO-OPA), generating250 mJ of eyesafe output.

[0046] Since other modifications and changes varied to fit particularoperating requirements and environments will be apparent to thoseskilled in the art, the invention is not considered limited to theexample chosen for purposes of disclosure, and covers all changes andmodifications which do not constitute departures from the true spiritand scope of this invention.

[0047] Having thus described the invention, what is desired to beprotected by Letters Patent is presented in the subsequently appendedclaims.

What is claimed is:
 1. An eyesafe, Q-switched, laser system for targetidentification, ranging, and gated viewing, said laser system having anumber of diodes for optical pumping, comprising: a resonant pumpederbium (RPE) laser having a storage lifetime that minimizes said numberof diodes needed to pump said Er laser, said RPE laser being in band toI² devices.
 2. The eyesafe, Q-switched laser system in accordance withclaim 1 , further comprising dilute concentrations of unsensitizedErbium in an approximate range between 0.1 and 2% of active ion, andhaving a lifetime of ˜10 msec for a 1.5 micron transition.
 3. Theeyesafe, Q-switched laser in accordance with claim 2 , furthercomprising an Erbium crystalline or glass host pumped by 1.5 microndiodes or diode pumped Yb-Er glass lasers.
 4. The eyesafe, Q-switchedlaser system in accordance with claim 1 , further comprising anenergy/pulse between ˜250 and 300 mJ.
 5. The eyesafe, Q-switched lasersystem in accordance with claim 1 , wherein said Erbium laser furthercomprises a wavelength of ˜1.5 microns.
 6. The eyesafe, Q-switched lasersystem in accordance with claim 2 , wherein said number of diodes pump˜30 to 60 W at 1.5 microns wavelength for ˜10 ms.
 7. An eyesafe,Q-switched, laser system or gain medium for target identification,ranging, gated viewing, and for amplifying fiber communications links,said laser comprising: a resonant pumped erbium (RPE) laser having astorage lifetime that minimizes said number of diodes needed to pumpsaid optical parametric oscillators, said RPE laser being in band I²devices, and that permits the attainment of gain coefficients of 0.5-1cm¹.
 8. The eyesafe, Q-switched laser system or gain medium inaccordance with claim 7 , further comprising dilute concentrations ofunsensitized erbium in a range of ˜0.1 and 2% of active ion, and havinga lifetime of ˜10 msec for a 1.5 micron transition.
 9. The eyesafe,Q-switched laser system or gain medium in accordance with claim 8 ,further comprising an erbium crystalline or glass host material pumpedby 1.5 micron diodes or Yb-Er glass laser.
 10. The eyesafe, Q-switchedlaser system or gain medium in accordance with claim 7 , furthercomprising an energy/pulse between ˜250 and 300 mJ and a gaincoefficient from 0.51 cm-1 cm⁻¹.
 11. The eyesafe, Q-switched lasersystem or gain medium in accordance with claim 8 , wherein said erbiumlasers further comprise a wavelength of ˜1.5 microns.
 12. The eyesafe,Q-switched laser system or gain medium in accordance with claim 8 ,wherein said diodes pump ˜30 to 60 W at 1.5 micron wavelength for ˜10ms.
 13. An eyesafe, Q-switched, laser system or gain medium for targetidentification, ranging, and gated viewing, said laser system having adiode array pump source, comprising: a plurality of diodes needed toachieve high energy storage and high gain.
 14. A gain medium suitablefor amplifying the output of Er fiber lasers to achieve sufficient powerfor reliable free space communications links, comprising: aresonant.pumped erbium laser having a storage lifetime sufficient toachieve high gain.
 15. The gain medium in accordance with claim 14 ,further comprising dilute concentrations of unsensitized Erbium in anapproximate range between 0.1 and 2% of active ion, and having alifetime of ˜10 msec for a 1.5 micron transition.
 16. The gain medium inaccordance with claim 14 , further comprising an Erbium crystalline orglass host pumped by 1.5 micron diodes or diode pumped Yb-Er glasslasers.
 17. The gain medium in accordance with claim 14 , wherein saidErbium laser further comprises a wavelength of ˜1.5 microns.